1. Field of the Invention
This invention relates to the field of manufacturing oxides of alkenes and the generation of peroxides for use in such manufacture. More particularly, it relates to an integrated, continuous, liquid phase process in which a first reaction generates hydrogen peroxide in organic solution; this reaction mixture is used for epoxidation of an olefin, which is also maintained in organic solution. When hydrogen peroxide is generated under the conditions disclosed and the reactions are coupled according to the invention, the reaction mixture may be used directly as a reagent for the second reaction. Thus there is no requirement for extraction or separation of hydrogen peroxide after the first step nor for removal of products of this first reaction such as ketones. The second reaction is maintained in liquid phase by the use of an organic solvent in combination with appropriate temperature and pressure conditions. This epoxidation reaction also requires the presence of a titanium-impregnated, amorphous silica catalyst, which is described below as a part of the invention.
2. Background of the Invention
Olefinic epoxides, such as ethylene oxide and propylene oxide, are soluble in organic solvents but insoluble in aqueous solutions. The production of epoxides from olefins results in commercially valuable products that have multiple uses in the plastics and chemical industries. The broad range of olefinis available as starting material is capable of generating epoxides which serve as intermediates in the preparation of a vast spectrum of chemical compounds.
The industrial epoxidation of olefins is performed by the reaction of the olefin with a peroxide reagent. Either hydrogen peroxide or an organic peroxide may be used, although the appropriate reaction conditions, the attainable reaction efficiency and the stability of these reagents varies substantially. If the peroxide reagent is generated from organic precursors as part of an integrated process of olefin epoxidation, economies may be realised by reducing the handling and storage of these chemicals.
The integrated process must couple two reactions: a first reaction that would generate peroxide efficiently within a liquid organic medium and a second reaction that would introduce the peroxide product to an olefin maintained in liquid phase by solvent under pressure. Successful integration of the two reactions requires that the conditions of each should be selected so that the products of the first reaction are compatible with the physical and chemical conditions of the second. Besides balancing effects of temperature and polarity upon solubility and stability, the reaction conditions must minimise undesirable secondary reactions of the peroxide with the various organic substrates, products and solvents employed in an integrated process. This is a particular problem if hydrogen peroxide is used as an reagent or intermediate product, generally requiring that the hydrogen peroxide is isolated from reagents and co-products before it can be used in the epoxidation reaction. Previously, the practicality of combining and integrating these reactions has been compromised by the relatively low yields of hydrogen peroxide obtained from the organic reactions known in the art and by the violent oxidising properties of hydrogen peroxide: although commercially available in aqueous solutions of up to 70% by weight, hydrogen peroxide is unstable and is capable of reacting explosively with organic materials.
Hydrogen peroxide may nevertheless be used as an oxidising agent for the production of epoxides from olefins, but it is especially desirable to generate this reagent at the time and the location in which it is required, in order to minimise hazardous handling and storage operations. Moreover, if the kinetic advantages of the strongly oxidising properties of hydrogen peroxide are to be exploited by using this reagent in the epoxidation reaction, its capacity to undergo side reactions with other components of the reaction mixture must be limited. Thus substantial benefits may be gained by integrating into a combined reaction process the production of hydrogen peroxide and its utilisation for olefin epoxidation. Yet the detrimental complications that such a procedure would generatexe2x80x94such as the additional potential for side-reactions in which the hydrogen peroxide may participatexe2x80x94must also be addressed.
Further advantages are to be derived by generating the hydrogen peroxide from organic reagents in organic solution: the reagent may then be mixed rapidly and efficiently with the olefin and the solvent, permitting the epoxidation reaction rate to be controlled accurately. The ability to exercise fine control over the rate of this exothermic reaction is a prerequisite for the development of continuous methods of olefin epoxidation, which offer the potential for additional economic benefits through increased efficiency, particularly when associated with an integrated production process.
The nature of the epoxidation catalyst is also critical for the success of the integrated process. A solid catalyst provides advantages of simple recovery, for example by filtration, prior to regeneration. The supporting structure for transition metal catalysts of epoxidation reactions fall into two categories, possessing either a porous, crystalline zeolite or silicalite structure which possess the additional properties of a xe2x80x98molecular sievexe2x80x99, or the structure of a solid, amorphous silica or alumina. Catalysts in the former group are expensive and limit the useful range of olefins with which they may be used because the diameter of the pores in the support limit access to the catalyst""s active centres. Catalysts of the latter group have been used with less reactive organic peroxides as oxidising agents, but if hydrogen peroxide is the oxidant they lower selectivity by expanding the spectrum of significant side reactions that occur, especially if reactions are combined in series. Substantially reduced efficiency and contaminated products are the result.
3. Description of the Prior Art
Industrial epoxidation processes are generally performed by introducing hydrogen peroxide or an organic peroxide to the olefin while the latter is dissolved in an organic solvent. Development of integrated processes in which the peroxide reagent is produced in situ has been discouraged by the technical difficulties outlined above.
Nevertheless, there have been disclosures of industrial olefin epoxidation procedures in which the peroxide reagent is generated immediately prior to the epoxidation reaction as part of an integrated process. These processes lack either an adequate solution to one or more of the technical problems addressed by the present invention, or suffer from another disadvantage, as indicated in more detail below.
In U.S. Pat. No. 5,214,168 Zajacek and Crocco disclose an integrated process of air oxidation of an aryl-substituted secondary alcohol, followed by the use of the oxidation product for epoxidation of an olefin in the presence of a crystalline titanium silicalite catalyst.
European Patent No. EP 0 526 945 discloses an integrated method of olefin epoxidation utilising hydrogen peroxide that is generated in situ. The hydrogen peroxide is produced from a redox reaction between oxygen or air and an alkylanthrahydroquinone. It then reacts with the olefin in the presence of a titanium silicalite catalyst and a specific mixture of organic solvents, comprising one or more aromatic hydrocarbons, one or more polar organic compounds of high boiling point and an alcohol of low molecular weight (methanol). The precise reasons for using this complex mixture of solvents are not disclosed in the publication. Yet the rationale may be related to the low solubility exhibited by alkylanthroquinones and alkylanthrahydroquinones when dissolved together: this characteristic limits the maximum quantity of hydrogen peroxide that can be generated by any specified volume of reactor.
Clerici and Ingallina, European Patent No. 0 549 013, disclose, inter alia, a process of olefin epoxidation by hydrogen peroxide in the presence of titanium silicalite that uses an aqueous mixture of alcohols as solvents for extracting the hydrogen peroxide generated in an alkylanthrahydroquinone redox system. However, the low solubility of the alkylanthrahydroquinones in the solvents used for the reaction significantly limits the commercial utility of this process.
Rodriguez and Zajacek, U.S. Pat. No. 5,463,090, disclose an integrated process for the production of epoxides based upon molecular oxygen oxidation of an alkylammonium salt of a sulphonic acid substituted anthrahydroquinone. The reaction product from the oxidation contains hydrogen peroxide and is used for olefin epoxidation in the presence of a titanium silicalite catalyst. Although the oxidation and epoxidation may be performed concurrently, the sulphonic acid-substituted anthraquinone alkylammonium salts are highly soluble in polar protic media such as water and lower alcohols. Consequently, the difference in solubility between these reagents and the olefin reactants, in the various solvents of interest, would be expected to substantially diminish the rate and extent of the reaction. Differential distillation is used to recover the aliphatic epoxide product.
U.S. Pat. No. 5,384,418, European Patent Applications No. EP 0 568 336 and European Patent Application No. EP 0 732 327 disclose procedures for the epoxidation of olefins in which the oxidation of secondary alcohols by oxygen or air generates hydrogen peroxide and the corresponding ketone. The resulting solution of hydrogen peroxide is used, following intermediate treatment, for olefin epoxidation in the presence of crystalline titanium silicalite catalyst, with methanol acting as solvent.
A discussion of the art relating to epoxidation catalysts is provided below, following the descriptions of the two reactions that are incorporated into the integrated process for producing olefin oxides.
Reviewing the art associated with the first reaction of the combined process, it is known that hydrogen peroxide may be generated from the oxidation of secondary alcohols by molecular oxygen:
O2+R.CH(OH).R less than = greater than H2O2+R.CO.R
When optimising the preparation of hydrogen peroxide from secondary alcohols the preferred product, besides the hydrogen peroxide itself, is the corresponding ketone. Thus the most commonly employed alcohol is isopropanol, with acetone being the major organic product: catalysts may be used but are not required for the industrial process. Yet in practice side reactions cause major difficulties when attempting to obtain high yields of hydrogen peroxide, and organic peroxides are generated as typical by-products.
Oxidation of isopropanol yields mixtures of organic peroxides and hydrogen peroxide: U.S. Pat. No. 2,869,989; U.S. Pat. No. 3,156,531; U.S. Pat. No. 3,294,266 and British Patent 758,907. Other secondary alcohols used as starting materials for hydrogen peroxide production include 1-phenylethanol and cyclohexanol: U.S. Pat. Nos. 2,871,102 to 2,871,104. Production of hydrogen peroxide by liquid phase oxidation of 1-phenylethanol with molecular oxygen, in which the hydrogen peroxide is recovered in organic solution, is disclosed in U.S. Pat. Nos. 5,254,326, 4,975,266 and 4,897,252. The oxidation of secondary alcohols with high boiling points such as diaryl methanols, in which the hydrogen peroxide is isolated in vapour form is described in U.S. Pat. No. 4,303,632.
Under industrially suitable conditions of temperature and pressure the production of hydrogen peroxide by oxidation of secondary alcohols with liquid-phase molecular oxygen is enhanced by admixing primary alcohols and/or ethers with the secondary alcohol. Not only is the reaction rate increased but also selectivity of the reaction towards hydrogen peroxide is improved. Thermal decomposition of hydrogen peroxide is decreased and side-reactions with other reactants are diminished. (Hydrogen peroxide is capable of reacting with both the secondary alcohols used as starting material and with the ketones produced during the oxidation reaction.) This approach to solving the problem of simultaneously achieving high reaction rates while retaining selectivity to hydrogen peroxide is disclosed in European Patent Application No. EP 0 839 760.
Reviewing the art relating to the second reaction of the combined process, it is known that the epoxidation of unsaturated olefinic compounds may be achieved with a broad range of reagents. Organic hydroperoxides may be used to obtain olefin epoxides in liquid phase: as an industrial process this generates the alcohol derivatives of the initial organic hydroperoxide as coproducts. Epoxidation of unsaturated olefinic compounds using hydrogen peroxide is another well-recognised reaction, which requires the use of a catalyst to be performed economically as an industrial process: 
Currently many of the catalysts developed for this reaction comprise synthetic zeolites: crystalline silicalite and/or alumina structures incorporating transition metals and their oxides. Crystalline silica is impregnated with titanium salts, such that titanium is incorporated into the crystal lattice. These titanium silicalite catalysts may require activation by being calcined in an oxidising atmosphere at elevated temperature to generate titanium oxide at the active centres. Catalysts designed to improve the efficacy of olefin epoxidation reactions are disclosed, for example, by U.S. Pat. Nos. 4,41,501, 4,666,692, 4,701,428, 4,824,976 and 4,833,260.
Although the selectivity for the epoxide is relatively high when using these crystalline titanium silicalite catalysts, non-selective opening of the epoxide""s oxirane ring also occurs during the reaction. Selectivity for epoxide may be augmented by treating the catalyst with an alkaline agent to neutralise the acid centres of the catalyst surface that are responsible for the undesirable ring-opening process: Clerici and Romano, EP 0 230 949. Non-basic salts such as lithium chloride, sodium nitrate and potassium sulphate suppress the oxirane ring opening mechanism and improve selectivity for the olefin epoxide: Crocco and Zajacek, EP 0 712 852.
Methanol is generally favoured as a solvent for the olefin epoxidation reaction because of its capacity to act as a proton donor, so that it is also considered to be a co-catalyst: M. G. Clerici et al., 1991, J. Catal. 129; 159. However, the use of this solvent is problematical in the epoxidation of propylene, specifically in the later stages of purifying the product, because of the proximity of its boiling point with that of propylene oxide. Alternative reagents and solvents permitting differential distillation may be employed, as disclosed in Deguchi, et al., EP 0 673 935.
Synthetic titanium derivatives of zeolite analogues, such as the crystalline silicalites, are known in the art and are frequently used for organic oxidation reactions using hydrogen peroxide. However, the restrictive pore diameter of the crystalline titanium silicalites (5.6xc3x975.3 xc3x85) denies access to the active centres of such catalysts by many larger olefin molecules and consequently epoxidation of these olefins, for example norbornene, will not be proceed efficiently using such catalysts.
Moreover, when used for epoxidation of olefins using hydrogen peroxide the catalytic activity of crystalline titanium silicalites is significantly more selective towards olefin epoxidation than is that of catalysts in which titanium is supported on amorphous, solid silica structures (as employed in the epoxidation reaction of the present invention). Of particular relevance to integrated processes of olefin epoxidation, the disclosure of Esposito et al. in U.S. Pat. No. 4,480,135 teaches that both primary and secondary alcohols are readily oxidised by hydrogen peroxide in the presence of a crystalline titanium silicalite catalyst. That such undesirable reactions are not observed under the restrictive epoxidation conditions of U.S. Pat. No. 5,124,168 indicates that the crystalline titanium silicalite catalysts may vary significantly in selectivity.
All the integrated olefin epoxidation processes described in the art and reviewed above employ for the epoxidation reaction crystalline titanium silicalite catalysts. In addition to the disadvantages associated with these zeolite analogues"" activity as molecular sieves, and their apparently variable selectivity, the complexity of synthesising such catalysts results in their commanding an elevated price. That the titanium catalysts also undergo rapid deactivation in the epoxidation reaction medium indicates the advantages to be gained from the development of an epoxidation catalyst that can be regenerated in a simple, industrial-scale process, either by basing it upon an inert agglomerate support, (e.g. EP 0 200 260) or by modifying its method of preparation (e.g. EP 0 638 362). An alternative would be to develop a catalyst that remains active for an extended period.
Epoxidation catalysts in which titanium is supported on solid amorphous silica are known to be effective in the epoxidation of olefins by organic hydroperoxides, as disclosed in, for example, U.S. Pat. No. 3,642,833, 3,923,843, 4,021,454 and 4,637,342. However, for industrial applications these catalysts have proven ineffective for the epoxidation of olefins using hydrogen peroxide as oxidising agent because they lack the requisite specificity and selectivity towards the desired olefin epoxide. Nevertheless, WO 94/23834 discloses synthesis of a product of silica and titanium fluoride under specific experimental conditions which catalyses a broad variety of chemical oxidation reactions, including the epoxidation of olefins by oxidation with either organic peroxides or hydrogen peroxide. As would be expected, this catalyst provides only a moderate selectivity towards epoxide. Surprisingly, however, undesirable side-reactions are not observed when the olefin epoxidation reaction is catalysed by the titanium-impregnated amorphous silica catalyst of the present invention. The selectivity towards epoxide that is displayed by the catalyst of the present invention is particularly unexpected given the broad spectrum of primary and secondary alcohols, ketones and solvents that may be present in the epoxidation step of the integrated process. This highly selective catalytic activity is obtainable by using the present invention""s particular method of preparing this robust catalyst of titanium supported on solid amorphous silica, which suggests that the method selected to prepare such catalysts may influence both the catalyst""s structure and the selectivity of its catalytic activity.
Alternative catalyst structures may incorporate alumina, which may be substituted with ammonium, alkali metal or alkaline earth metal ions, in order to increase accessibility to the active centres or to reduce the oxirane ring-opening side reactions, although selectivity for epoxide may be compromised: U.S. Pat. No. 5,412,122, 5,374,747, and 5,621,122, EP 0 659 685.
Although individual methods are known in which one or other of the two component reactions can be performed in liquid phase, a combination of such liquid phase methods is perceived to be highly desirable: i.e. a first liquid phase stage in which hydrogen peroxide is generated through the oxidation of a secondary alcohol by molecular oxygen followed by a second liquid phase stage in which the hydrogen peroxide reacts with the olefin, which must normally be dissolved in an organic solvent to remain liquid. A combined two-step method would provide economic and logistical advantages of reduced handling and storage of intermediate reactants, particularly in view of the chemical instability of hydrogen peroxide and its potentially hazardous nature.
Yet in order for such a combined process to operate effectively and economically on an industrial scale optimum efficiency would be required from the individual component reactions so that yields of the appropriate products from each of the reactions were maximised. If either reaction were performed under sub-optimal conditions, not only would the generation of by-products from the first step contaminate the subsequent reaction and substantially reduce the yield obtained from the second step, but poor efficiency in the second step would jeopardise the industrial applicability of the combined process.
4. Objects of the Invention
The invention addresses the requirement for a convenient, safe, effective and economical method for generating olefin epoxides from alkenes by providing solutions to the difficulties associated with the implementation of a continuous process which integrates the generation and delivery of hydrogen peroxide with its utilisation for olefin epoxidation.
The invention couples into a single compatible process a first step which is a convenient, effective and economical method for generating hydrogen peroxide in organic solution at high concentration and a second step, an epoxidation reaction that exploits a robust catalyst capable of high selectivity for epoxide with the selected reactants and solvents. Physical conditions for both reactions are selected to permit the combined process to be operated entirely in liquid phase.
By combining these reactions into a single, integrated process the invention must overcome the following problems:
a. the differential solubility of reagents; polarity differences between various reagents and the organic solvents required to maintain the product in solution;
b. the selectivity of peroxide production; the difficulty in obtaining a high percentage of hydrogen peroxide in solution from reactions involving oxidation of organic substrates and employing organic solvents;
c. the reactivity of hydrogen peroxide and its tendency to participate in multiple side-reactions with the organic reagents, products and solvents present;
d. the inefficiency and expense of isolating the peroxide reagent from contaminating alcohol reagents and ketone products before using it in the epoxidation reaction;
e. selective catalysis of the epoxidation step; the requirement for a catalyst capable of catalysing the epoxidation of olefinis by hydrogen peroxide in organic solvents while minimising acceleration of alternative oxidation reactions between hydrogen peroxide and the multiple organic components present in the integrated epoxidation reaction mixture.
Thus in order to combine the reactions required for olefin epoxidation into a single process the invention addresses, inter alia:
i. in the first step, the need to obtain elevated yields and rates of reaction when generating hydrogen peroxide from an organic reagent, such as a secondary alcohol;
ii. the selection of solvent systems for the two reactions that are mutually compatible and which maintain all the reagents in miscible solutions; and
iii. in the second step, the need for a catalyst that accelerates the epoxidation reaction itself without catalysing undesirable side reactions of the hydrogen peroxide to a significant extent.
Improving the rate and selectivity of the first reaction enables the resulting organic solution to be used to introduce hydrogen peroxide directly into the second reaction, the epoxidation, in a form in which it is miscible with the organic solvent required to maintain the olefin reactant in the liquid state. Moreover, no intermediate purification steps are necessary when the two reactions are performed under the specified, compatible combination of conditions. The ability to use the organic reaction mixture from the first step directly as a source of hydrogen peroxide for the second, epoxidation reaction requires the development of an appropriately selective catalyst that will minimise the reaction of hydrogen peroxide with the other reagents and reaction products present, including those introduced with the addition of the first reaction mixture. Simplicity and cost-effective preparation and regeneration are additional beneficial characteristics for such a catalyst.
The adaptation of the combined reactions to a continuous, integrated process is achieved by the technological developments described herein and their incorporation into a single, sequentially organised and compatible procedure.
The present invention provides for a combination of reactions enabling alkene oxides to be produced in a two part, liquid-phase process. By performing the entire process in liquid phase, without the requirement for the purification or enrichment of intermediates or for the removal of by-products, the invention facilitates adaptation of this integrated process for olefin epoxidation to methods of continuous industrial production.
The invention provides for a continuous, integrated process for producing an organic epoxide comprising the following steps: (i) oxidation of a secondary alcohol (reagent A) by molecular oxygen or air; (ii) epoxidation of an olefin by admixing the reaction mixture of step (i) with a solution of the olefin in an organic solvent (reagent B) in the presence of a titanium catalyst supported on amorphous silica at temperatures comprising between 50xc2x0 C. and 140xc2x0 C., wherein the titanium catalyst is obtainable by impregnation of silica having a surface area comprising between 50 m2/g and 900 m2/g with a solution of titanium alkoxide and/or titanocene in oxygenated organic solvent (reagent C).
In a further embodiment the invention provides for the rate of the first reaction to be enhanced by the presence of a primary alcohol and/or ether, which also greatly improves selectivity toward hydrogen peroxide. Under these circumstances the reaction may be performed efficiently under moderate temperatures and pressures in the presence of an organic solvent. This solvent maintains the hydrogen peroxide in solution as it is generated so that it may be rapidly and effectively mixed with the organic solution in which the epoxidation reaction occurs.
The second reaction is performed under pressure and at moderately elevated temperatures in the presence of organic solvent. The invention provides for the use of a defined catalyst for the epoxidation, composed of titanium-impregnated, solid, amorphous silica, which enables this reaction to proceed with the desired efficiency and selectivity toward the olefin epoxide. The titanium catalyst is obtainable by the impregnation of silica having a surface area comprising between 50 m2/g and 900 m2/g with a solution of titanium alkoxide and/or titanocene in oxygenated organic solvent. Additionally, other elements could be added to the silica such as germanium, vanadium, etc. These elements may be incorporated onto silica in a prior, separate step from the impregnation with titanium, or the silica may be impregnated with all components together using a combined solution, or subsequently in a separate step after the impregnation with titanium. The resulting titanium-impregnated silica is isolated as a robust, solid, amorphous residue and may be activated by calcination or by treatment with solvents, either prior to use, or for purposes of regenerating used catalyst after recovery from the epoxidation reaction mixture.
In particular, the invention provides a safe, effective and efficient means of producing propylene oxide from two organic solutions, one of which contains propylene and the other being an organic solution of reagents that includes hydrogen peroxide. The latter solution is generated from the oxidation of a secondary alcohol such as 1-phenylethanol by molecular oxygen or air. These two reactions are coupled in a continuous, entirely liquid-phase process.